Knowledge of the interplay between the morphology, composition and optical appearance of biological photonic systems can provide broad inspiration for novel artificial photonic elements. Nature's most vivid colors, highest transparencies, strongest whites and deepest blacks rely on ordered, quasi-ordered or disordered structures with lattice constants or scattering element sizes on the order of the wavelength of visible radiation. By inducing interference or diffraction, biological photonic structures of a wide structural diversity strongly alter the spectral composition of reflected and transmitted light resulting in the stunning structural colors of many organisms. One-dimensional multilayer arrangements play an important role in the creation of structural colors in nature and have primarily been studied in the animal kingdom, especially the insect world. Planar layered photonic system have recently also been increasingly frequently reported in various plants.
The fruits of the plant Margaritaria nobilis in the rain forests of Middle and South America have a striking blue-green hue. The plant partly relies on seed dispersal by birds which might be attracted by the colorful display. The cells in the fruit's blue seed coat are elongated and mostly appear blue or green. Several layers of cells are stacked on top of each other with varying planar orientation of the individual cell layers. The interior volume is occupied by a periodic concentrically-layered morphology with an overall periodicity of (180±30) nm. Light incident on the fruit's surface undergoes interference within the periodic structure in each cell resulting in the reflection of blue light.
Under directional illumination a planar multilayer interference structure can only display its bright coloration in the specular reflection direction. The hue of such planar Bragg stacks strongly depends on the angle of incidence. Under diffuse illumination the observed color blue-shifts for increasing observation angle. By contrast, in M. nobilis fruits the superposition of a microscopic curvature on the nanoscale regularity of the layered structure within each individual seed coat tissue cell combined with the fruit's overall macroscopic curvature leads to an increased visibility of the reflected structural color across a wide angular range. Under directional illumination, a part of the curved multilayer in a majority of the individual cells is oriented to satisfy the specular reflection condition providing a spatially varying pixelated sparkle of different hues that depend on the locally varying angle of light incidence. In diffuse light this sparkle is suppressed as the light reflected by each fruit cell towards the observer originates from light incident on the fruit from various directions, producing a more isotropic color that only gradually changes across the fruit due to the macroscopic curvature.
The emergence of unique structural and optical properties from combinations of structures on different length scales within hierarchical synergistic assemblies is a principle often encountered when studying natural systems. Increasingly frequently this concept is also applied in novel optical technologies. The hierarchical photonic architecture in the seed coat of the M. nobilis fruit is the key element involved in the creation of intense blue and green hues. It provides inspiration for the manufacture of artificial photonic fibers with the optical functionality being defined by the interplay of nanoscopic regularity and superimposed microscopic curvature.
Optical fiber systems with multilayer claddings have theoretically been discussed since the late 70's and have more recently been manufactured using standard fiber drawing processes from macroscopic preforms. The choice of materials that can be drawn into multilayer fibers is constrained to a limited, albeit continuously expanding set of components. In particular, the preform material combinations need to provide an appropriate refractive index contrast and have matching thermal expansion coefficients in order to prevent fracturing at the material interfaces during the processing at elevated temperature.
Planar flexible multilayer systems using soft organic materials has been reported. The planar sheets were used for the creation of multilayer claddings on rods with macroscopic diameter to facilitate the manufacture of microscopically planar multilayer stacks.